I know that we now have fluconazole to treat bryopsis. I've even successfully treated it using this method.
However, I just wanted to share some information I have in case folks prefer the old method of raising Mag using Tech M or a more natural way.
Kent Tech M worked for Bryopsis before the formula was changed. Continuum Reef Basis Magnesium is identical to the old original Kent Tech M formula and will work for Bryopsis when used to raise the Magnesium level up to very high levels. This method can be stressful to some corals however.
Bacter Clean-M has had good success eradicating Bryopsis, as it has other nuisance algaes, and that is what I would recommend that you use first. It does take some time, like in excess of a month of use and sometimes over 2 months.